atomic mass number

a·tom·ic mass num·ber

(ă-tom'ik mas nŭm'bĕr)

The mass of the atom of a particular isotope relative to hydrogen 1 (or to one twelfth the mass of carbon 12), generally very close to the whole number represented by the sum of the protons and neutrons in the atomic nucleus of the isotope; it is not to be confused with the atomic weight of an element, which may include a number of isotopes in natural proportion.

atomic mass number

A number equal to the sum of all the neutrons and the protons in an atom's nucleus.

See: mass number

See also: number

a·tom·ic mass num·ber

(ă-tom'ik mas nŭm'bĕr)

The mass of the atom of a particular isotope relative to hydrogen 1 (or to one twelfth the mass of carbon 12), generally very close to the whole number represented by the sum of the protons and neutrons in the atomic nucleus of the isotope; it is not to be confused with the atomic weight of an element, which may include a number of isotopes in natural proportion.